The present invention relates to an optical system for use with recording and reading information on an optical information medium, and particularly to an optical system, for use with recording and reading information on an optical information medium, the optical system comprising a coupling lens for an objective lens having a large numerical aperture.
Recently, in accompanied with a rapid growth of a recording and reading apparatus for an optical information medium, such as a compact disk (CD), it is greatly desired that the cost of the optical system of the apparatus is lowered, and the size of the optical system is made smaller, and therefore, various improvements are conducted on the optical system. The results of the technical improvements of the optical system are shown in FIG. 2.
The numerical aperture (NA) of an objective lens for the CD is lower than 0.45. Emitted light beams from a light source 4, such as a semiconductor laser or the like, are in parallel through a collimator lens 6, and an image is formed on the optical information medium 1 by the objective lens. Initially, as shown in FIG. 2(a), manufacture of the objecting lens with a single lens was tried (for example, refer to Japanese Patent Publication Open to Public inspection No. 76512/1982). However, since a collimator lens 6 comprises two spherical lenses, it is further desired to reduce the cost of the collimator lens.
According to the progress of manufacturing engineering of the objective lens, the following optical system was developed in which: as shown in FIG. 2(b), the collimator lens is removed; and an image of a light beam flux from the light source is directly formed on the optical information medium by one objective lens having an object point and an image point at a definite distance (for example, refer to Japanese Patent Publication Open to Public Inspection No. 56314/1986). Recently, this type objective lenses are used for many CDs.
Further, the following proposal has been made: in order to reduce the total length of the optical system and to increase the utilization efficiency, a coupling lens 3 comprising a spherical single lens is used in the optical system as shown in FIG. 2(c); image formation magnification of the coupling lens is made larger than zero; and wave surface aberration is lowered to that of the conventional collimator lens (refer to Japanese Patent Publication Open to Public Inspection No. 43842/1987).
In the optical system for recording and reading information which is recently being developed, the most popular optical system is structured with one objective lens as shown in FIG. 2(b). However, the NA of the optical system to which this optical system can be applied, is smaller than 0.45, and when the NA is greater than 0.47, the optical system shown in FIG. 2(a) must still be used.
When the NA is greater than 0.47, and if it is desired to use the optical system shown in FIG. 2(b), the following problems occur.
Initially, the distance from the light source to the optical information recording surface (distance between the object and image) U is determined by design conditions. However, practically, deviation .DELTA.U of the distance U is caused by deflection of the optical information medium. At the present time, in this type of apparatus, when the deviation .DELTA.U occurs within an available magnification ratio, the optical system is controlled such that the objective lens is moved along the optical axis and the image is automatically focused on the surface of the optical information medium. However, aberration is caused by the aforementioned deviation.
The aberration consists mainly of spherical aberration, and an amount of which is proportional to the fourth power of the NA. The amount of the aberration is further proportional to .DELTA.U, and to the function in which an absolute value .vertline.m.vertline. of a lateral magnification ratio of the optical system is variable.
When the amount of the aberration caused by .DELTA.U is defined as Wu, and the function in which the absolute value .vertline.m.vertline. of the lateral magnification ratio is variable, is defined as .alpha.(.vertline.m.vertline.), then the .alpha.(.vertline.m.vertline.) is a monotone increasing function of the variable .vertline.m.vertline., and the Wu is expressed by the following equation: EQU Wu=.alpha.(.vertline.m.vertline.).multidot.(NA).sup.4 .multidot..DELTA.U(1)
Although this aberration also occurs in the case where the NA of the optical system for the CD is approximately 0.45, the level of aberration is negligible. However, in the case where the NA of the optical system is more than 0.47, the aberration can not be ignored.
When an automatic focusing operation is conducted under the condition that the objective lens is integrated with the light source, the foregoing problem does not occur. However the apparatus surrounding the optical system becomes complicated and cost is unavoidably increased.
In the case of a resin objective lens, the influence due to its temperature characteristics must be considered as another problem.
Generally, when temperature is changed, a change in the refractive index of a resin lens is 10 times as large as that of glass lens. Accordingly, in the case of the resin lens, when there is a temperature difference .DELTA.T between a reference temperature in design and that of a practical environment, the aberration Wt occurs.
This aberration mainly includes spherical aberration, and is proportional to the fourth power of the NA, a focal distance f, .DELTA.T, and a monotone increasing function .beta.(.vertline.m.vertline.) in which an absolute value .vertline.m.vertline. of the lateral magnification is variable. EQU Wt=.beta.(.vertline.m.vertline.).multidot.(NA).sup.4 .multidot.f.multidot..DELTA.T (2)
Even when lenses with the same focal distance and NA have the same .DELTA.U or .DELTA.T, an aberration amount Wu or a value of Wt changes due to the lateral magnification of the optical system.
For example, an optical system for a video disk reading apparatus is assumed as an example, in which the lens has the NA of 0.5 and a focal distance of 1 mm.
Here, the distance between the object and the image can be expressed as follows. EQU U={.vertline.m.vertline.=(1/.vertline.m.vertline.)+2}.multidot.f+HH'.(3)
Where, m is a lateral magnification ratio of the optical system, f is a focal distance, HH' is a distance between a principal point on the light source side of an objective lens and that on the optical disk side of the lens.
Here, the thickness on the axis of the objective lens is defined as 0.81 mm, the refractive index of material is 1.486, and a change of the refractive index due to temperature is -12.times.10.sup.-5 /+1.degree. C. When .vertline.m.vertline. is changed, changes of respective aberration amounts Wt and Wu under the condition that .DELTA.T=30.degree. C., and .DELTA.U=+1 mm, are shown in FIG. 3. It can be seen from FIG. 3 that: the more .vertline.m.vertline. is increased, corresponding increase in the aberration occurs.
That is, in the case where the objective lens is glass, the aberration W is caused by .DELTA.U, and EQU W=Wu (4)
In the case where the objective lens is resin, the aberration W is the sum of the aberration caused by .DELTA.U and that caused by .DELTA.T, and EQU W=Wu+Wt (5)
As can be seen in FIG. 3, as the lateral magnification ratio is decreased, the influence of .DELTA.T and .DELTA.U is decreased. However, for example, when the lateral magnification ratio is m&gt;-1/6, the NA of the light source side is decreased, and a more powerful laser is necessary for obtaining the required light amount. Further, a certain length of operation distance is necessary, and the distance between an object and an image of the optical system becomes impractically long. In order to make the optical system compact, it is required that the light beam is reflected by a reflection mirror, and this, in turn, increases the cost of the optical system. Conversely, when the lateral magnification ratio is decreased (an absolute value of the lateral magnification ratio is increased), the influence of the change of aberration W is correspondingly increased.
Practically, this influence is discussed on the example of the optical system structured as in FIG. 2(b). For example, a light collecting optical system for a recording and reading optical system for use in a magneto-optical (MO) disk is supposed here, and the objective lens of the optical system having the following characteristics: the NA 0.53, the lateral magnification ratio m=-1/5, and the distance between the object and image U=30 mm, will be considered as follows. Here, the lens is resin in which the refractive index n=1.5031, the lens thickness is 3.25 mm, and the reference design temperature is 25.degree. C. This lens data will be shown in a following table as a comparative example (conventional example). In that table, data of the disk G will also be shown. The aberration of the lens is shown in FIG. 4.
If this optical system can be used as the foregoing light collecting optical system as is, the cost can be greatly reduced compared with the optical system shown in FIG. 2(a). However, as described above, the aberration due to .DELTA.U and .DELTA.T is a problem which must be considered.
Changes of the wave front aberration are shown in FIG. 5 at the time when .DELTA.U is changed from -1 mm to +1 mm in cases where .DELTA.T is -30.degree. C., 0.degree. C. and +30.degree. C.
In FIG. 5, the following can be clearly seen: when .DELTA.T is -30.degree. C. and .DELTA.U is -1 mm, an rms value of the wave front aberration is 0.10 .lambda.ms; when .DELTA.T is +30.degree. C. and .DELTA.U is +1 mm, an rms value of the wave front aberration is 0.095 .lambda.ms (in each case, .lambda.=780 nm); and in both cases, the value of the wave front aberration is larger than the Marechal allowance value even in the axial wave front aberration, and there is the possibility that the performance of the system can not be effective in the environment in which the system is used.
In practice, considering allowances for other characteristics, the value of the aberration is required to be no more than 0.055 .lambda.ms when .DELTA.T is within the range of .+-.30.degree. C. and .DELTA.U is .+-.1 mm, in order to realize the performance of the system.
Based on the above reason, conventionally, for the optical system having a large NA such as a light collecting optical system for a recording and reading optical system which is used for a video disk, mini disk, magneto-optical disk, phase change disk and the like, the optical system in which a collimator lens is provided as shown in FIG. 2(a) is in popular use. However a very expensive lens such as an aspherical lens or a spherical glass lens comprising 1 group 2 element lenses is used for this collimator lens, thereby, the apparatus is more expensive than a compact disk player.
The following optical system has been disclosed in Japanese Patent Publication Open to Public Inspection No. 43,842/1987: a coupling of spherical single lenses is used for reducing the total length of the optical system and allowing for increase in the efficiency of use of the light source beam; and when the imaging magnification of the coupling lens is larger than zero, a value of the wave front aberration is lowered to approximately the same as that of the conventional collimator lens. However, a high NA optical system having an NA more than 0.47 has not been considered yet. Specifically, variations of temperature, and variations of the distance between an object and an image have not been considered yet, resulting in difficulty in realizing a large NA optical system.
An object of the present invention is to obtain an optical system for recording and reading information which has sufficient performance even with variations of temperature and variations of the distance between the object and the image, while maintaining the desired imaging magnification ratio and the distance between the object and the image.